Good question. I actually considered using "prawns" in the title, but I figured most people seeing this would be American (like me) and might not know what prawns are. I suspect one of two reasons: 1) they're not the same animal, so if what I bought actually was shrimp, then selling it as "shrimp" would be correct; or 2) because the only term I've seen/ heard decapods marketed as is "shrimp" in the US, so that's what people here call both species. Also, more often, at least near where I live (southern US, not near a coast), we don't often see the larger sizes, so even if someone were served something prawn-size, it would blow our minds, lol.